Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle span and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles which alternate vehicles using fossil fuels such as gasoline vehicles and diesel vehicles. These electric vehicles generally use nickel-metal hydride secondary batteries as power sources. However, a great deal of study associated with use of lithium secondary batteries with high energy density and discharge voltage is currently underway and some of them are commercially available.
Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
Among these cathode active materials, LiCoO2 is the most generally used owing to superior physical properties such as long life span and good charge/discharge characteristics, but has low structural stability and is costly due to natural resource limitations of cobalt used as a raw material, thus disadvantageously having limited price competiveness.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages of superior thermal stability and low costs, but have disadvantages of low capacity and bad low-temperature characteristics.
In addition, LiMnO2-based cathode active materials are relatively cheap and exhibit higher discharge characteristics, but are disadvantageously difficult to synthesize and are unstable.
In order to solve the afore-mentioned problems, the present invention provides a low-cost highly functional cathode active material comprising lithium transition metal composite oxide wherein constituent elements satisfy the requirements including predetermined composition and oxidation number conditions, as mentioned below.
In this regard, U.S. Pat. No. 6,964,828 discloses a lithium transition metal oxide having a structure of Li(M1(1-x)Mnx)O2 wherein M1 is a metal other than Cr, and each Ni has an oxidation number of +2, each Co has an oxidation number of +3, and each Mn has an oxidation number of +4, provided that M1 is Ni or Co.
In addition, Korean Patent Laid-open No. 2005-0047291 suggests a lithium transition metal oxide wherein Ni and Mn are present in equivalents amounts and have an oxidation number of +2 and +4, respectively.
As another example, Korean Patent No. 0543720 discloses a lithium transition metal oxide wherein Ni and Mn are present in substantially equivalent amounts, Ni has an oxidation number of 2.0 to 2.5 and Mn has an oxidation number of 3.5 to 4.0. This patent discloses that Ni and Mn should substantially have an oxidation number of +2 and +4, respectively, and that lithium batteries are deteriorated in properties, unless Ni and Mn have an oxidation number of +2 and +4, respectively, as apparent from Examples and Comparative Examples.
Also, Japanese Patent Application Publication No. 2001-0083610 discloses a lithium transition metal oxide which is represented by a structure of Li((Li(Ni1/2Mn/2)(1-x))O2 or Li((Lix(NiyMny CoP)(1-x))O2 and contains Ni and Mn in equivalent amounts. In accordance with the application, provided that Ni and Mn are present in identical amounts, Ni and Mn form Ni2+ and Mn4+, respectively, realizing structural stability and thus providing the desired layered structure.
Accordingly, in accordance with the prior art, the average oxidation number of transition metals should be +3, which is mentioned in U.S. Pat. No. 7,314,682. In this patent, the inventors disclose lithium transition metal oxide represented by the structure of Li(2+2x)/(2+x)M′2x(2+x)/(2+x)M(2−2x)/(2+x)O2-δ wherein M′ is an element having an average oxidation number of +3, in which M′ is not Li, and M is a transition metal having an average oxidation number of +3.
As can be confirmed from the afore-mentioned related patents, it was conventionally believed that (i) transition metals should have an average oxidation number of +3 in order to impart a stable layered structure to lithium transition metal oxide, and (ii) Ni present in an amount equivalent to Mn4+ should have an oxidation number of +2 in order to impart superior electrochemical properties to the lithium transition metal oxide, based on the premise (i).
However, the inventors of the present application confirmed that, in the case where Mn4+ and Ni2+ are simply selected to obtain an average oxidation number of +3, Ni2+ or the like is transferred to reversible Li sites, the problem, deterioration in electrochemical properties, cannot be solved.
Meanwhile, U.S. Pat. Nos. 7,078,128 and 7,135,252 suggest materials wherein Mn is present in an amount higher than that of Ni. However, the inventors of the present invention confirmed that a high amount of Mn cannot change an oxidation number of Mn4+ upon Li-charging and thus causes a decrease in capacity.